1. Field of the Invention
The invention relates to a method for controlling an electronically slip-controllable vehicle brake system.
2. Description of the Prior Art
The vehicle brake system described is capable automatically, that is, independently of what the driver is asking for and thus independently of an actuation of the foot pedal 12, of performing a braking event. In automatic braking events, the switchover valve 44 is switched by the electronic control unit 28 into the blocking position, and thus the existing hydraulic communication of the master cylinder 14 with the wheel brakes 20, 22; 24, 26 is interrupted. The buildup of the brake pressure is effected solely by a triggering of the drive motor 39 of the pressure generator 38. If during an automatic braking event of this kind the driver additionally actuates the foot pedal 12, for instance because he wants to build up a higher brake pressure than is predetermined by the system, then the driver disadvantageously perceives a pedal actuation characteristic (characteristic force/travel curve) that is altered compared to the normal service brake operating mode. The driver senses a harder, less-resilient pedal than in typical service braking events, because of the brake pressure in the vehicle brake system that has already been built up by the pressure generator 38, in combination with the switchover valve 44, kept in the blocking position, and also because of the relatively slight supply quantity of pressure fluid from the pressure generator 38 via the open high-pressure switching valve 42.
Electronically slip-controllable vehicle brake systems are prior art. Known vehicle brake systems are capable of braking individual wheels of the vehicle regardless of what the driver is asking for, for instance to bring a vehicle into a stable driving state thereby (Electronic Stability Program or ESP), to eliminate slip at the driving wheels (Traction Control or TC), or to perform adaptive control of the speed or the following distance from a vehicle ahead (Automatic Cruise Control or ACC). The structural layout, known per se, of such vehicle brake systems is shown in FIG. 1.
Accordingly, an electronically slip-controllable vehicle brake system includes a brake booster 10, actuatable by the driver by foot pedal 12, with a downstream master cylinder 14. Two identically constructed hydraulic brake circuits 16, 18 are connected to the master cylinder 14. Each brake circuit 16, 18 serves to actuate two wheel brakes 20, 22; 24, 26 each. Modulation of the brake pressure prevailing in these wheel brakes 20-26 is possible by means of magnet valves, which are triggerable by an electronic control unit 28. For that purpose, each wheel brake 20-26 is preceded hydraulically by a pressure buildup valve 30 making a pressure buildup possible and followed hydraulically by a pressure reduction valve 32 making a pressure reduction possible. The outlet of the pressure reduction valves of a brake circuit 16, 18 discharges into a common return line 34. A pressure fluid reservoir 36 is connected to this return line 34. The return line 34 leads to the intake side of a pressure generator 38, which is actuatable by a triggerable drive motor 39 and which delivers pressure fluid at elevated pressure to the pressure buildup valves 30 of the wheel brakes 20-26 and to the master cylinder 14.
The intake side of the pressure generator 38 continues to communicate with an intake line 40. The latter connects the pressure generator 38 to the master cylinder 14, in the event that the volume of pressure fluid furnished by the pressure fluid reservoir 36 should not suffice for required brake pressure buildup. The intake line 40 is controllable by a so-called high-pressure switching valve 42.
Each brake circuit 16, 18 is also equipped with a so-called switchover valve 44. It is connected between the master cylinder 14 and the pressure buildup valve 30 and serves, in the event of an automatic braking event, to decouple the master cylinder 14 hydraulically from the wheel brakes 20-26.
The magnet valves, in the electronically nontriggered state, assume their basic positions shown in FIG. 1. The high-pressure switching valve 42 and the pressure reduction valves 32 of a brake circuit 16, 18 are then located in a blocking position, while the switchover valve 44 and the pressure buildup valves 30 assume their through or open position. Upon an actuation of the foot pedal 12 by the driver, the buildup of a brake pressure in the wheel brakes 20-26 by muscle power is thus possible.
For modulating the brake pressure, for instance if one of the wheels of the vehicle is tending to lock because of an applied excessive brake pressure, the pressure buildup valve 30 of the brake circuit 16, 18 of the affected wheel is triggered by the electronic control unit 28 and switched over to its blocking position. At the same time, the pressure reduction valve 32 is put in its open position, and the drive motor 39 of the pressure generator 38 is triggered. Pressure fluid from the affected wheel brake 20-26 can as a result flow out into the pressure fluid reservoir 36, which until then is empty, and be temporarily stored there. At the same time, pressure fluid from this pressure fluid reservoir 36 is aspirated by the pressure generator 38. A pressure reduction in the wheel brake 20-26 is effected until such time as the locked wheel resumes rotary motion. This can be ascertained from wheel rotation sensors (not shown), which deliver their measurement signals to the electronic control unit 28. The aspirated pressure fluid is compressed by the pressure generator 38 and prestored in the closed pressure buildup valves 30 of a brake circuit 16, 18 or fed back into the master cylinder 14. Thus it is again available for an ensuing brake pressure increase.
The vehicle brake system described is capable automatically, that is, independently of what the driver is asking for and thus independently of an actuation of the foot pedal 12, of performing a braking event. In automatic braking events, the switchover valve 44 is switched by the electronic control unit 28 into the blocking position, and thus the existing hydraulic communication of the master cylinder 14 with the wheel brakes 20, 22; 24, 26 is interrupted. The buildup of the brake pressure is effected solely by a triggering of the drive motor 39 of the pressure generator 38. If during an automatic braking event of this kind the driver additionally actuates the foot pedal 12, for instance because he wants to build up a higher brake pressure than is predetermined by the system, then the driver disadvantageously perceives a pedal actuation characteristic (characteristic force/travel curve) that is altered compared to the normal service brake operating mode. The driver senses a harder, less-resilient pedal than in typical service braking events, because of the brake pressure in the vehicle brake system that has already been built up by the pressure generator 38, in combination with the switchover valve 44, kept in the blocking position, and also because of the relatively slight supply quantity of pressure fluid from the pressure generator 38 via the open high-pressure switching valve 42.
Different pedal actuation characteristics (characteristic force/travel curve) of the foot pedal 12, which are also dependent on the particular operating state of the vehicle brake system, make the metering of the brake pressure more difficult and can annoy the driver.